1. Field of the Invention
The present invention relates to a method of removing nitrogen oxides from combustion exhaust gases. More particularly, it relates to a highly efficient method of removing nitrogen oxides from combustion exhaust gases formed in industrial combustion apparatus.
2. Description of the Prior Art
The nitrogen oxides NO and NO.sub.2 (hereinafter referring to as NO.sub.x) are toxic to the human body. These oxides are absorbed by the human body and body functions deteriorate as a result. NO.sub.x also causes photochemical smog. Accordingly, it is important to develop a method for removing NO.sub.x from gases discharged into the atmosphere. Of particular importance is the removal of NO.sub.x from the combustion exhaust discharged from steam generating boilers for electric power plants which are a major source of nitrogen oxides.
Various methods for removing NO.sub.x from combustion exhaust gases have been proposed. For example, it has been known to remove NO.sub.x by the catalytic reduction of the NO.sub.x containing combustion exhaust gas at about 200.degree.-500.degree. C. with ammonia in the presence of various metal oxides supported on a carrier as the catalyst. When the concentration of oxygen is relatively high, it is possible to selectively react the NO.sub.x with ammonia. However, in this method it is necessary to use an expensive catalyst which is poisoned by the various impurities such as sulfur oxide, steam, soot and the like in the exhaust combustion gas to lower the catalytic activity.
Accordingly, it is necessary to treat the catalyst to reactivate and recover it. It is necessary to use expensive apparatus to perform the process. Further, operating costs are high because of the necessity for compensating for the pressure loss at the catalyst layer and adjusting the temperature of gas to be the optimum temperature for the reaction. The temperature is different depending upon type of the catalyst, and the range of the applicable temperature is narrow. For example, the active temperature in the case of a platinum catalyst is 210.degree.-350.degree. C.
It has also been proposed in U.S. Pat. No. 3,900,554 to remove NO.sub.x from the NO.sub.x containing exhaust combustion gas by maintaining said gas at 700.degree.-1300.degree. C. in the presence of oxygen and ammonia or an ammonia precursor without using a metal oxide catalyst. The NO.sub.x is reduced to nitrogen and the process has significant industrial advantages. The inventors have conducted experiments to apply this method to various combustion apparatus, and have found the following facts:
In industrial combustion apparatus such as a boiler for electric power generation and other power plants, it is usual to dispose a high density of heat exchange pipes made of stainless steel in the high temperature zone of the exhaust combustion gas stream in order to attain a high heat efficiency. When the ammonia source is fed to the zone having a high density of metallic heat exchange surfaces, the ammonia is decomposed and converted by the catalytic action of the metallic surface to form NO.sub.x. Accordingly, in industrial combustion apparatus, it is important to select a position for feeding the ammonia to limit the density (heat transfer area) of the metallic heat exchange surfaces downstream thereof. Moreover, when a fuel such as heavy oil containing a vanadium component has been used in the combustion apparatus such as a boiler, a semi-molten scale containing the vanadium component adheres on the heat exchange surfaces. Typically, the heat exchange surface is disposed in a zone having a temperature of about 700.degree. C. whereby the vanadium scale causes the decomposition of the ammonia which is fed in at a temperature higher than 700.degree. C. so as to cause NO.sub.x formation. Accordingly, when a heat exchange surface covered with a scale containing a vanadium component is disposed downstream near the position for feeding ammonia, it is desirable to limit the heat transfer area of the heat exchange surface.
There is, then, a need for a technique which would allow for the addition of ammonia to exhaust gases without the occurrence of ammonia decomposition on the heat exchange surfaces.